Protected or protecting semiconductor switches are increasingly employed to protect downstream components, such as traces on circuit boards or PCB (printed circuit board) traces or cables, and thus to take over a protection function. Complete employment or employment of all protection elements in a protection circuit for downstream components by semiconductor switches is, however, problematic particularly in an overload case due to the increased response times, as will still be explained later.
So as to adapt the response time of a conventional semiconductor switch, so-called smart semiconductor switches, which enable adjusting a load current threshold leading to immediate switch-off via a current sense function, are employed for protection, for example. These conventional semiconductor switches, for example, are made by the company International Rectifier offering a smart semiconductor switch or smart high-side switch IR 3310 in its portfolio. Moreover, conventional semiconductor switches supporting software-side programming of a load current threshold and a fading time, such as the products MC33982 or MC33984, are offered by the company Freescale within the scope of its Extreme Switch family.
In the conventional semiconductor switches, in which an overload current threshold is adjusted during operation, or an overload current threshold fixed prior to startup, it is disadvantageous that the semiconductor switches lose their conductivity when the load current exceeds the load current threshold.
Moreover, semiconductor switches in which overtemperature protection is implemented are offered, these semiconductor switches interrupting a current through a downstream component, such as a cable, when its temperature exceeds a certain temperature threshold. Due to the great thermal capacity of a package of a protecting semiconductor switch and the thermal capacities resulting from the arrangement of the package of the semiconductor switch on the PCB, a response of the protecting semiconductor switch is substantially more inert than that of a comparable protection, particularly in an overload case. By an overload case, a state in which a voltage greater than a nominal voltage for which the components of the circuit are designed is applied to a circuit, or a current greater than a nominal current for which the components of the circuit are designed flows into the circuit is understood.
Particularly in an overload case, when the load current is only slightly greater than the nominal current, a more inert response of the protecting semiconductor switch than that of the cable to be protected may occur.
In FIG. 8, response times for two protection elements and one cable are shown. The current flowing through a protecting element or a cable is plotted on the x-axis in amperes, whereas the response time is plotted on the y-axis in seconds. After the response time, the protection elements inhibit current flow through the protection element, or the cable will become destroyed.
Curve 11 shows a course of a response time of a cable in dependence on a current through the cable at a temperature of 85° C. Curve 13 explains a course of a response time of a semiconductor switch, here of the BTS6143 type, at a temperature of 85° C. in dependence on a current through the semiconductor switch, whereas curve 15 shows a course of a response time of a protection in dependence on a current through the protection. Here, the protection is a mini-fuse designed for 10 amperes. It can be seen from FIG. 8 that the response time of the conventional semiconductor switch is higher than the response time of the cable in a critical region 17. This leads to the fact that the cable becomes destroyed before the conventional semiconductor switch inhibits the current through the cable downstream thereto. Thus, the cable is not reliably protected by the semiconductor switch over the entire range of the current values illustrated in FIG. 8.